Modeling infection of spring onion by Puccinia allii in response to temperature and leaf wetness.

The influence of temperature and leaf wetness duration on infection of spring onion (Japanese bunching onion) leaves by Puccinia allii was examined in controlled-environment experiments. Leaves of potted spring onion plants (Allium fistulosum cv. Yoshikura) were inoculated with urediniospores and exposed to 6.5, 10, 15, 22, or 27 h of wetness at 5, 10, 15, 20, or 25 degrees C. The lesion that developed increased in density with increasing wetness duration. Relative infection was modeled as a function of both temperature and wetness duration using the modified version of Weibull's cumulative distribution function (R(2) = 0.9369). Infection occurred between 6.5 and 27 h of leaf wetness duration at 10, 15, 20, and 25 degrees C and between 10 and 27 h at 5 degrees C, and increased rapidly between 6.5 and 15 h of wetness at 10, 15, and 20 degrees C. At 25 degrees C, few uredinia developed regardless of the wetness duration. Parameter H, one of eight parameters used in the equation and which controls the asymmetry in the response curve, varied markedly according to the temperature, so that the model could be improved by representing H as a function of wetness duration (R(2) = 0.9501).